High Octane fuel-Rip off on a 2-stroke?

Redrider1991

The other day my friend told me he was running high octane fuel in his everyday car to clean it out, (Which I'm pretty sure it doesn't clean it out) and I have been told that it can burn your valves up. So I warned him and he said that is was fine to run high octane so I looked it up.

I was always under the impression that the octane rating was the explosive potential of the fuel, but it's actually a rating on how much compression the fuel can take before ignition.

So since two stroke engines run at much lower compression than 4-strokes wouldn't high octane fuel be a waste of money?

There's probably something else that factors in that I couldn't find, but maybe we'll all save some money now!

Zakrad_u

jbird710

Many high-performance engines are designed to operate with a high maximum compression and thus demand high-octane premium gasoline. A common misconception is that power output or fuel mileage can be improved by burning higher octane fuel than a particular engine was designed for. The power output of an engine depends in part on the energy density of its fuel, but similar fuels with different octane ratings have similar density. Since switching to a higher octane fuel does not add any more hydrocarbon content or oxygen, the engine cannot produce more power.

zig06

Really, unless something was built for it, using higher octane fuel is just a waste of money. Most cars and trucks today are built to run on the cheap stuff. Only consider using mid grade or premium if you are getting spark knock. As an example, a truck pulling a large trailer up a big hill in the middle of the summer may have a spark knock, so that would be a reason to use premium.

As far as dirt bikes go, if your bike is stock normal premium pump gas will be all you need. Only a bump in compression and port work would require a higher octane number to prevent any possible spark knock.

DoggyDaddy

Premium gas might have more or better additives than regular to prevent or clean carbon deposits (or might not). If you don't have a detonation (ping, spark knock) issue then I'd say that premium is a waste of money whether 2 stroke or 4 stroke.

OK, I am really dating myself here, but once upon a time I road raced an RD250 Yamaha. For those (like most everybody reading this) unfamilair with that model it was a smaller bore brother to the RD350, a popular sporty bike in the 1970s, and THE bike for the 400 Production road racing class. Anyway, I got the 250 because my then GF didn't want to ride it anymore and I ran it in 250 Production. Thinking that I would get an edge from running premium fuel I spent the extra money (OK at that time it was not much difference in cost). When the bike started to run poorly I found lead fouling on the plugs, actually accumulated out of the fuel. Went back to regular and never had that problem again.

Of course, these days there isn't any lead, but still my point is that unless you need it the extra octane does no good.

screamin injin

Only use the higher octane with high compression engines, otherwise it is a waste, but have you seen some of the high-grade gases? Shell's is almost completely clear while the lower-grade is a golden brown. They filter out a little more "trash" from the "good" stuff, which they should do for all of their gas.

Redrider1991

KJ790

An engine performs the best on the lowest octane you can run without it detonating. All stock MX bikes are fine won premium pump gas (and some with lower octane than that). If your engine has been modified it may need slightly higher octane. Remember that mixing oil with the gas (like on a two stroke) lowers the octane rating of the fuel significantly, so even though they may be lower compression than a four stroke, they often require around the same octane of fuel.

tjscott2012

if you want to go with your manual, it says always run over 90, but it all depends on elevation and climate. if your running an athena high comp kit which is ridiculous, like 14:1 then you will need to start bumping up the octane.

Smidgy

So could I even run standard since 2t's are lower compression or should I stick to running 93?

I would start with what they want you to in the manual, when you mix your oil into the gasoline your are then lowering your octane rating(usually atleast 2 pts). You need the oil to provide the compression in a 2 stroke(It provides the seals around th piston rings as well as lubricates the engine) The amount of oil you run will be the determing factor on this. The more oil you run the higher the compression to a maximum that the engine is capable of. That is why you see usually two values on compression for a two stroke, it is harder to create that seal at high rpm's therefore the comprssion goes down. If you run too much oil then you decrease your octane level so greatly that you then limit the power making capability(potential energy in the fuel) of the gasoline, so there is a bit of a balance there. If you run oil rich mix say 25:1 to 30:1 then I would run 93 octane. If you were running somewhere around say 35:1 to 40:1 I would try to run 91 octane. It is always good practice to start with the manuals reccomendations, but it has no gaurantee that it is going to be the best setup for you, you may have different air density, fuel quality, oil quality than they do in giving you those numbers. A little trial and error will set you in the right direction, part of the fun in dirtbikes is finding the best setup so you can beat the next guy with the same bike...lol:banana:

MXkid714

Smidgy

There are two values for a 2 stroke because one value is when the exhaust valve is closed and the lower one is when the exhaust valve is open.

I am not saying your wrong but that is not what I have always seen, Everytime I have seen two values one is for low RPM and the other is for high RPM, if the exhaust valve is open then there is effectively no compression(because it is exhausting), So I am unsure what you are trying to say maybe I just don't understand, the compression is built up when all valves are closed not open

x1glider

When the bike started to run poorly I found lead fouling on the plugs, actually accumulated out of the fuel. Went back to regular and never had that problem again.

Low compression engines running high octane fuel usually have a problem burning the fuel completely since the higher the octane, the more heat/spark energy it takes to ignite it.

I see this in air cooled Harley Davidson engines all the time. HD riders tend to run premium in their 9:1 CR engines for no apparent reason other than that's what other misinformed people tell them. It just spreads like an infection...the bad info. I see caked up deposits on the piston crown, combustion chamber and valve faces and sometimes see rings cemented to the piston grooves. All because the fuel isn't completely burned. I run a modded HD, 10.5:1 CR, and only put 91 octane in it in Texas 100+ weather and I don't knock. There's a lot of other things these other people are doing wrong too with carb tuning and ignition timing.

For daily commuting, I run 87 octane in my BMW R1100S which is 11:1 CR and don't have knocking issues. Being oil cooled instead of air cooled might have a little to do with it. But when I go somewhere where I run the piss out of it and bang off the limiter all day, I run the high octane, just in case. Bikes that are built to breathe better at higher rpms need to have their volumetric efficency taken into account. You'll need the extra octane at those rpms to keep detonation away.

Point is, the lower octane will burn more complete and leave less deposits behind in normal riding. Riding at the rpm limits, you'll need higher octane. 2 strokes can get away with lower octane because of the oil in the premix keeping the engine running cooler.

There is also zero more power potential in 93 than 87. No gain to be had.

CBus660R

I am not saying your wrong but that is not what I have always seen, Everytime I have seen two values one is for low RPM and the other is for high RPM, if the exhaust valve is open then there is effectively no compression(because it is exhausting), So I am unsure what you are trying to say maybe I just don't understand, the compression is built up when all valves are closed not open

You need to understand how the exhaust valve works on a 2-stroke. It doesn't open and close like a 4-stroke valve. What it really is is a device that raises or lowers the height of the exhaust port. The height of the exhaust port and when the piston covers it up determines the CR of a PV equipped 2-stroke . At low RPMS, the exhaust valve is down, lowering the height of the ex port and raising the CR. When the PV "opens" it actually is raising up, giving you a higher ex port and lower CR.

TTurner161

You need to understand how the exhaust valve works on a 2-stroke. It doesn't open and close like a 4-stroke valve. What it really is is a device that raises or lowers the height of the exhaust port. The height of the exhaust port and when the piston covers it up determines the CR of a PV equipped 2-stroke . At low RPMS, the exhaust valve is down, lowering the height of the ex port and raising the CR. When the PV "opens" it actually is raising up, giving you a higher ex port and lower CR.

Yeah what he said. Smidgy, I think you may have been confused when the term exhaust valve was used instead of powervalve.

Smidgy

You need to understand how the exhaust valve works on a 2-stroke. It doesn't open and close like a 4-stroke valve. What it really is is a device that raises or lowers the height of the exhaust port. The height of the exhaust port and when the piston covers it up determines the CR of a PV equipped 2-stroke . At low RPMS, the exhaust valve is down, lowering the height of the ex port and raising the CR. When the PV "opens" it actually is raising up, giving you a higher ex port and lower CR.

thanks man I understand perfectly what he was saying now, definately a little confused with the exhaust valve term, that makes sense, thanks for setting me on the right track.

One2nine

Nah I have standard compression. I wouldn't mind getting a high comp kit though. Is it a nice power gain without alot of reliability loss?

I was just reading about them, says it tends to flatten out the top-end so I probably wouldn't like it

On a yz125?!?!?!

Keep in mind the new yzf's are running 13.5's stock on 93 pump gas. With moderate head work running race fuel will definitely benefit you. Race fuel benefit's lots of motor's, but if you're stock or close to it, you're wasting your hard earned money. The same relatively goes for 2 strokes as well. With wild porting/heard work you are pretty much required to run an increased octane. Also keep in mind that oil mixed in gas lowers your octane rating.

brewster

So could I even run standard since 2t's are lower compression or should I stick to running 93?

You also have to consider static and dynamic compression ratios in a 2 stroke. Static being measured from where the piston closes the ports and dynamic takes into consideration the scavenging and ram effect of the 2 stroke tuning which will give you a higher effective compression ratio.